As the semiconductor industry continues to mature, cost pressures persist that drive companies to continually reduce manufacturing costs. A direct result of this pricing pressure is the movement to smaller geometry fabrication processes with reduced feature sets. A consequence of the reduced process feature set is the removal of dedicated (non-substrate) bipolar devices that would require extra processing steps, and therefore cost, to implement. Note bipolar devices typically exhibit substantially smaller (and more predictable over temperature) offset voltages and have less noise when compared with MOS (metal-oxide-semiconductor) devices. However since dedicated bipolar devices are not available in most reduced feature set processes of today, MOS devices must typically be used. The larger and less predictable device mismatch levels in MOS devices result in larger and less predictable circuit performances both for initial tolerances and drift over temperature. Additionally, increasing relative noise levels in circuits using MOS devices are exacerbated by reductions in process line width due to thinner gate oxides. The increased noise levels and larger voltage shifts over temperature resulting from MOS devices are un-desirable features in a voltage reference.
A standard substrate bandgap reference current uses a pair of diode connected substrate bipolar junction transistors with different current densities to develop a proportional-to absolute-temperature (PTAT) voltage (ΔVBE) across associated resistors. Though there are several other contributors to the temperature variation in the reference e.g. transistor temperature coefficients, differential temperature coefficients in the resistors, VBE curvature, and accuracy of the bandgap voltage, the dominant factor, and the one addressed by this invention, is the offset and drift of the amplifier when an MOS amplifier is used. Like all bipolar devices, the buried junctions of the substrate bipolar junction transistors have a relatively small response to package stress and typically match closely during fabrication. Layout techniques enhance this behavior and initial matching errors can be trimmed by adjusting the output voltage of the reference. The resistor-to-resistor temperature coefficient variation benefits from all the same layout techniques that improve resistor matching and can typically be reduced to the point where it is not an issue. VBE curvature, a nonlinearity in the transistors that results in an undesired shift in the reference voltage over temperature, can be reduced to acceptable levels using one of many known correction techniques. The bandgap voltage is usually very stable on a given process and is not typically the limitation for a reference design. This leaves the non-idealities of the MOS amplifier, input referred offset, temperature drift, and noise, as the dominant error sources in the reference. The input referred offset and noise voltage of the MOS amplifier are gained up by an approximate factor of (1+R2/R3) to the output of the reference. Though this gain can be minimized by increasing the PTAT voltage, practical limitations on the ratio of current densities in the substrate bipolar junction transistors place the gain factor (on a single bandgap) in the 8X-12X range. Note, this means that a 1 uV/° C. drift in the amplifier results in almost a 10 ppm/° C. drift in the reference. Thus, random drift offsets, and low frequency noise of the MOS amplifier are the main impediment for achieving a tight temperature coefficient specification for the reference.